Monitoring of Fatigue in Welded Beams Using Piezoelectric Wafer Based Impedance Technique

Real-life aerospace and mechanical structures are mostly prone to fatigue, which is the occurrence of localized but progressive damage due to continuous fluctuating stresses. Fatigue damage can be monitored by observing changes in the structural stiffness resulting from strength reduction as a function of the number of loading cycles. The problem is more severe if the welds are subjected to such fatigue behavior. This paper employed non-destructive testing (NDT) method such as dye-penetrant and advanced NDT method such as the piezoelectric wafer based electromechanical impedance (EMI) technique to study the fatigue behavior of fillet welds in three specimens. Two specimens were tested in one single day, whereas the third on three consecutive days by unloading the specimen at the end of each day. Generally in laboratory, fatigue test is conducted by continuous cyclic loading/unloading till failure. However, the third specimen was subjected to intermittent fatigue to understand the stiffness changes which are due to the crack and its growth, or material realignment and partial strength restoration. Furthermore, cracks leading to failure are captured using EMI based signature analysis in these specimens. The signatures were further analyzed using statistical index, numerical modeling and peak shift measurements. The uniqueness of this paper is to present a comprehensive study of both load and non-load carrying welds by means of intermittent fatigue tests, using a thorough admittance signature analysis. The major observation of this study is that the fatigue behavior can be viewed as fusion of axial and transverse load changes coupled with crack severity, as observed from signatures.